Preparation of ammonium paratungstate via adding (NH4)2CO3 or NH4HCO3 to ammonium metatungstate solution

Abstract

A new technology was proposed to produce ammonium paratungstate (APT) from ammonium metatungstate (AMT) solution by adding (NH4)2CO3 or NH4HCO3 in order to reduce energy consumption and subsequent ammonia recovery burden in crystallization step. Specifically, the effects of ammonium source dosage, temperature, reaction time and stirring speed on crystallization yield, crystalline phase and morphology of APT products were systematically investigated. The results showed that crystallization yields under optional conditions with (NH4)2CO3 and NH4HCO3 as ammonium sources could reach 85.4% and 86.9% with particle size (D50) of 358.8 μm and 441.3 μm, respectively. The crystallization mechanism could be identified as H2W12O40^6- first transforming to H2W12O42^6- and finally to H2W12O42^10-, resulting in the APT precipitation by H2W12O42^10- combining with NH4^+. (NH4)6[H6W12O42]·10H2O played as an intermediate in the crystallization, which could also react with ammonium sources to form APT crystals. Compared to NH3·H2O as an ammonium resource, the corresponding maximum crystallization yields under the same optimal conditions were in order of NH4HCO3>(NH4)2CO3>NH3·H2O, while different ammonium sources affect the morphology of crystallization product.

1. Introduction

Tungsten is an important strategic resource known as “industrial tooth” due to its excellent physical and chemical properties including high melting point, high strength, high density, etc [1−3]. It is widely used in many important industrial fields, such as oil drilling, national defense, aerospace, machinery manufacturing, special steel and new materials [4,5]. The raw materials of tungsten in nature are mainly wolframite and scheelite. Ammonium paratungstate ((NH4)10[H2W12O42]·nH2O, APT) is a vital intermediate product during tungsten extraction processes, which is mainly used to prepare the downstream products of tungsten oxide, tungsten powder, and tungsten carbide [6−8].

Generally, APT preparation process from minerals can be divided into alkaline method and acid method according to different decomposition treatments, where alkaline method dominates the industrial manufacture of APT products [9−12]. Alkaline method usually uses Na2CO3 or NaOH as a lixiviant to leach tungsten ores with obtaining a crude Na2WO4 solution and CaCO3 or Ca(OH)2 residues [13−16]. This process is carried out at high temperatures (≥180 °C) and consumes 2.5−5.0 times stoichiometric amount of alkaline reagent, producing a large amount of high-salt sodium-containing wastewater. The crude Na2WO4 solution is then processed via a solvent extraction or ion exchange step to prepare (NH4)2WO4 solution [17,18]. However, the molybdenum in Na2WO4 solution or (NH4)2WO4 solution must be removed by sulfide precipitation [19]. During solvent extraction and ion exchange steps, approximately 20−30 t and 80−120 t of wastewater are discharged for producing per ton APT [20,21]. Finally, APT is crystallized from purified (NH4)2WO4 solution by the evaporative crystallization.

Obviously, the current industrial APT production technology has the disadvantages of large auxiliary reagent consumption, huge high-salt wastewater discharge, high production cost and serious environmental pollution. In order to reduce NaOH or Na2CO3 consumption and increase WO3 leaching yield, YANG et al [22] used a mixture of Na3PO4, NaOH and CaF2 to leach low-grade scheelite by converting scheelite to Ca5F(PO4)3 residue and Na2WO4 solution. GONG et al [23] ro...